Image forming apparatus and scanning unit

ABSTRACT

After having been reflected by a first return mirror, a light beam deflected by a polygon mirror is caused to travel toward a second return mirror disposed in a position below a base section of the polygon mirror by way of an opening. An image forming apparatus is configured so as to satisfy a relationship of a&lt;b and another relationship of c&lt;b, given that an optical path length from a reflection surface of the polygon mirror to a reflection surface of the first return mirror is taken as “a”; another optical path length from the reflection surface of the first return mirror to a reflection surface of the second return mirror is taken as “b”; and still another optical path length from the reflection surface of the second return mirror to the surface of the photosensitive member is taken as “c.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus and ascanning unit, and more particularly, to an image forming apparatusequipped with a scanning unit which scans and exposes the surface of aphotosensitive member by means of emitting a light beam, as well as to ascanning unit provided in the image forming apparatus.

2. Description of the Related Art

An electrophotographic image-forming apparatus, such as a laser printer,forms an image by means of: deflecting a light beam emitted from alight-emitting section including a light source, such as a semiconductorlaser or the like, by a deflector such as an polygon mirror; scanningand exposing a uniformly-charged surface of the photosensitive member tothus form a latent image; rendering the latent image visible with toner;and transferring a toner image on a recording medium, such as arecording sheet or the like.

A scanning apparatus that exposes and scans the surface of thephotosensitive member by emitting a light beam requires highly accurateassembly in relation to the light-emitting position of a light sourcesuch as a semiconductor laser, the position of a polygon mirror, and theposition of a scanning optical system, or the like, which guides a lightbeam to a photosensitive member.

As described in, e.g., JP-A-2004-163463 (see FIG. 1), when aconventional scanning unit is mounted on an image forming apparatus, arigid-body plate, such as a steel plate, is usually disposed above aprocess unit equipped with a photosensitive drum or the like, in adirection orthogonal to the paper transport direction, to thus place thescanning unit above the rigid-body plate. In many cases, the light beamdeflected by the deflector, such as a polygon mirror, is reflected bythree return mirrors and guided to the surface of the photosensitivemember.

SUMMARY OF THE INVENTION

If a printer can be disposed in a space which is not very large; e.g., aspace on a desk, convenience is afforded to a user who uses a printer inan ordinary household. Hence, considerably strong demand has arisen forminiaturizing an image forming apparatus, such as a laser printer.However, as described in connection with JP-A-2004-163463, when thelight beam is guided to the photosensitive member through use of threereturn mirrors, difficulty is encountered in miniaturizing (particularlyslimming down) the scanning unit to a given size. This in turn posesdifficulty in miniaturizing (particularly, shortening a height of) anapparatus.

The present invention provides an image forming apparatus and a scanningunit, which can be miniaturized to a great extent.

According to an aspect of the present invention, there is provided animage forming apparatus including: a photosensitive member; a scanningunit that exposes and scans a surface of the photosensitive member byemitting a light beam, the scanning unit including a unit frame that hasa base section having an opening and an outer peripheral wallsurrounding a rim of the base section; a deflector that is disposed onan upper surface side of the base section and deflects the light beamtoward the outer peripheral wall; a first reflection optical elementthat is disposed with the opening being interposed between the firstreflection optical element and the deflector and reflects the lightbeam, which has been deflected by the deflector and travels toward theouter peripheral wall, toward a position below the base section; and asecond reflection optical element that is disposed on a lower surfaceside of the base section and reflects the light beam, which travelsafter having been reflected by the first reflection optical element andpassed through the opening, toward the photosensitive member; wherein anangle between the light beam entering the first reflection opticalelement and the light beam reflected from the first reflection opticalelement toward the second reflection optical element is an acute angle;and a relationship of a<b and another relationship of c<b are satisfied,given that an optical path length from a reflection surface of thedeflector to a reflection surface of the first reflection opticalelement is taken as “a”; another optical path length from the reflectionsurface of the first reflection optical element to a reflection surfaceof the second reflection optical element is taken as “b”; and stillanother optical path length from the reflection surface of the secondreflection optical element to the surface of the photosensitive memberis taken as “c.”

This configuration requires only two reflection optical elements, suchas return mirrors. The angle formed between the light beam entering thefirst reflection optical element and the light beam reflected from thefirst reflection optical element toward the second reflection opticalelement is made an acute angle, and the optical path length from thefirst reflection optical element to the second reflection opticalelement is made long. Thereby, a sufficient optical path length can alsobe ensured. Accordingly, an attempt can be made to shorten the distancebetween the scanning unit and the photosensitive member, therebyminiaturizing (particularly, shortening the height of) the image formingapparatus.

According to another aspect of the invention, there is provided ascanning unit that exposes and scans an object of scanning by emitting alight beam, including: a unit frame that has a base section having anopening and an outer peripheral wall surrounding a rim of the basesection; a deflector that is disposed on an upper surface side of thebase section and deflects the light beam toward the outer peripheralwall; a first reflection optical element that is disposed with theopening being interposed between the first reflection optical elementand the deflector and reflects the light beam, which has been deflectedby the deflector and travels toward the outer peripheral wall, toward aposition below the base section; and a second reflection optical elementthat is disposed on a lower surface side of the base section andreflects the light beam, which travels after having been reflected bythe first reflection optical element and passed through the opening,toward the photosensitive member; wherein an angle between the lightbeam entering the first reflection optical element and the light beamreflected from the first reflection optical element is an acute angle;and a relationship of a<b is satisfied, given that an optical pathlength from a reflection surface of the deflector to a reflectionsurface of the first reflection optical element is taken as “a” andanother optical path length from the reflection surface of the firstreflection optical element to a reflection surface of the secondreflection optical element is taken as “b.”

According to the image forming apparatus and the scanning unit, theoptical path of the light beam in the scanning unit is formed into anessentially wedge-shaped geometry. By means of this, only two reflectionoptical elements, such as return mirrors, are required. Consequently,this is advantageous in miniaturizing and slimming down the entirescanning unit. Further, an optical path length between the firstreflection optical element and the second reflection optical element ismade long, so that a sufficient optical path length can be ensuredwithin the scanning unit. Hence, an attempt can be made to shorten thedistance between the scanning unit and the photosensitive member, whichin turn yields an advantage of the ability to miniaturize the overallimage forming apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be more readily described with reference tothe accompanying drawings:

FIG. 1 is a perspective view showing the appearance of a laser printeremployed as an example image forming apparatus according to anembodiment of the present invention;

FIG. 2 is a schematic side cross-sectional view of the laser printer;

FIG. 3 is a top view for describing the configuration of a scanningunit;

FIG. 4 is a schematic cross-sectional view of the scanning unit;

FIG. 5 is a perspective view for describing a case where a unit frame isfixed to a rigid-body plate;

FIG. 6 is a perspective view for describing how a rear cover is attachedto the back of the unit frame; and

FIG. 7 is a perspective view for describing a case where a cylindricallens is attached to the back of a base section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A deflector in an embodiment of the present invention is disposed at oneof two opposing ends of an upper surface of a base section, and a firstreflection optical element is disposed at the other end of the two ends.Further, a second reflection optical element is disposed via the basesection at the one end of the two ends and a position below thedeflector. This configuration is effective for miniaturizing the overallscanning unit.

The scanning unit can include a first scanning lens interposed betweenthe deflector and the first reflection optical element; and a secondscanning lens interposed between the first reflection optical elementand the second reflection optical element. More specifically, thedeflector preferably has a configuration such that the light beam isdeflected as a result of the reflection surface rotating about a rotaryshaft; an opening is preferably formed in an area where the firstscanning lens and the second scanning lens are disposed; and the firstscanning lens and the second scanning lens are preferably provided so asto partially overlap each other in the opening along a direction of therotary shaft. When the second scanning lens is caused to approach thephotosensitive member; particularly, when the second scanning lens is acylindrical lens which gathers a light beam in a sub-scanning directionintersecting the main scanning direction, the width of the secondscanning lens in the sub-scanning direction must be increased, which isdisadvantageous in miniaturization of the scanning unit. Placing thescanning lens at the above-described position is effective forminiaturizing the scanning unit by means of slimming-down of thescanning lens.

The thickness of the cylindrical lens in the sub-scanning direction canbe set to about 6 mm or less. Further, an angle between the light beamentering the first reflection system optical element and a light beamhaving been reflected from the first reflection system optical elementcan be set to about 10° to 30°. The smaller angle is effective forslimming down the scanning unit.

The two ends of the second scanning lens can be respectively attached totwo attachment sections provided on a lower surface of the base section,by way of attachment members. An attempt can be made to slim down thescanning unit by means of attaching not the center of the secondscanning lens, but both ends of the same to the attachment sections.

Preferably, a positioning section of the second scanning lens is placedon either one of the two attachment sections. This is intended forpreventing deterioration of image quality, which would otherwise becaused when the second scanning lens swells as a result of an increasein humidity or temperature. The second scanning lens is formed fromresin by means of injection molding. When a gate section which is leftin a resin injection section after molding is provided at one end of thesecond scanning lens, the positioning section is preferably provided onan attachment section to which an end opposite the one end is to beattached.

The scanning unit preferably assumes a substantially wedge-shapedgeometry whose part close to the deflector is longer than a part closeto the first reflection system optical system in a heightwise directionwhen a rim portion of the unit frame is viewed from the side.

The image forming apparatus preferably further includes: a process unitincluding the photosensitive member and a development device forrendering visible a latent image formed by means of exposing andscanning the photosensitive member. When a reclosable cover is providedon the side surface of a housing opposing the first reflection systemoptical element, a space through which the process unit passes can beeasily assured by means of the essentially wedge-shaped geometry. As aresult, attachment and removal of the process unit to and from the imageforming apparatus by way of an opening formed when the cover is openedbecome easy.

An embodiment of the present invention will be described with referenceto the drawings.

(1) Overall Configuration of a Laser Printer

FIG. 1 is a perspective view showing the appearance of a laser printer 1according to an embodiment of an image forming apparatus.

The laser printer 1 shown in FIG. 1 has a top cover 18 which is tobecome an upper surface, and four side surfaces 2 a, 2 b, 2 c, and 2 d(side surfaces 2 c and 2 d are situated in positions where they remaininvisible in FIG. 1). A portion of the top cover 18 is receded into ahousing 2 to thus form a sheet output tray 72. A paper feed cassette 6capable of housing a plurality of sheets of recording medium, such asrecording paper, is detachably provided in a lower part of the housing 2so as to be pulled out from a front side surface 2 a of the housing 2. Amanual feed tray section 11 to be used for individually setting arecording medium and a reclosable front cover 16 are provided in thefront side surface 2 a.

As indicated by phantom lines in FIG. 1, a pair of side frames 190 b and190 d are provided on mutually-opposing inner side surfaces 2 b and 2 d,and a rigid-body plate 190, such as a steel plate, is provided betweenthe side frames 190 b and 190 d at a position below the top cover 18that serves as an upper surface of the housing 2. Each of the sideframes 190 b, 190 d is formed from a steel plate or molded from resin,such as polystyrene or ABS (acrylonitrile-butadiene-styrene).

The configuration of the laser printer 1 will be described in moredetail with reference to FIG. 2.

FIG. 2 is a schematic side-cross sectional view of the laser printer 1when viewed from the side surface 2 d.

The laser printer 1 includes, within the housing 2 having the top cover18 serving as an upper surface, the front cover 16 provided on the frontside surface 2 a, and a rear cover 60 provided on the rear side surface2 c, a paper feed section 3 for feeding recording paper serving as arecording medium, or the like, a process cartridge 4 for forming a tonerimage which is a visible image on the fed recording medium, a fixingunit 5 for fixing on the recording medium the toner image formed on thesame, and a paper output section 70 for outputting the recording mediumhaving passed through the fixing unit 5. In the specification, among thefour side surfaces 2 a to 2 d of the housing 2, a side surface on whichthe front cover 16 is to be mounted (i.e., a right side surface in FIG.2) is taken as the front side surface 2 a, and a side surface oppositeto the front side surface is taken as the rear side surface 2 c. Thefront side surface 2 a and the rear side surface 2 c are located on bothsides in the recording medium transport direction.

The paper feed section 3 includes the paper feed cassette 6, paper feedrollers 7, 8 disposed at positions above the leading (front side) end ofthe recording mediums stacked in the paper feed cassette 6 in therecording medium transport direction, and a paper feed pad 9. A paperfeed path 10 is formed in the paper feed section 3, wherein the paperfeed path is a recording medium transport path which inverts therecording medium fed from the paper feed cassette 6 and transports therecording medium to a lower portion of the process cartridge 4. Thepaper feed section 3 is provided with a pair of registration rollers 12facing the paper feed path 10. In addition to the recording mediumloaded in the paper feed cassette 6, a recording medium (recording paperor the like) set in the manual feed tray section 11 is also fed to thepaper feed path 10. In either case, after having been temporarilystopped by the pair of registration rollers 12, the recording medium issupplied to an image forming section of the process cartridge 4 at animage formation timing of the process cartridge 4.

The paper feed cassette 6 is provided below the process cartridge 4 andthe fixing unit 5 and detachably attached from the front side surface 2a of the housing 2. A paper press plate 13 and a spring 14 are providedwithin the paper feed cassette 6. The paper press plate 13 enableslaminated stacking of recording mediums, and an end of the paper pressplate 13 distant from the paper feed roller 7 is supported in apivotable manner, and an end of the same proximate to the paper feedroller 7 is vertically movable. Further, the spring 14 is provided so asto upwardly urge the rear surface of the end of the paper press plate 13proximate to the paper feed roller 7. Therefore, as the amount ofstacked recording mediums increases, the paper press plate 13 is pivoteddownwardly against the restoration force of the spring 14 while the endsection of the paper press plate 13 distant from the paper feed roller 7is taken as a fulcrum.

The paper feed roller 8 and the paper feed pad 9 are provided so as tooppose to each other, and the paper feed pad 9 is pressed against thepaper feed roller 8 by means of a spring 15 provided on the back of thepaper feed pad 9. The top sheet of the recording mediums stacked on thepaper press plate 13 comes into contact with and is pressed against thepaper feed roller 7 by means of the spring 14 from the back of the paperpress plate 13. The top recording medium is nipped between the paperfeed roller 8 and the paper feed pad 9, and is separated on a per-sheetbasis by means of the paper feed roller 8 and the paper feed pad 9 as aresult of rotation of the paper feed roller 8. The recording medium isfed to the paper feed path 10.

The recording medium supplied from the paper feed cassette 6 or themanual feed tray section 11 is supplied to the pair of registrationrollers 12 disposed at the positions above the paper feed roller 7 orthe like. After having registered the fed recording medium, the pair ofregistration rollers 12 transport the recording medium to the imageforming position (i.e., a position where a photosensitive drum 37 and atransfer roller 39 remain in contact with each other) within the processcartridge 4. The front side surface 2 a of the housing 2 is providedwith the front cover 16. The front cover 16 is provided which can beopened and closed with respect to the housing 2. The process cartridge 4is removably inserted by way of an opening which appears when the frontcover 16 is opened.

A scanning unit 100 provided at the position above the process cartridge4 includes a laser diode 271 which emits a laser beam (see FIG. 3); apolygon mirror 110 which is rotated at high speed by means of a polygonmotor 112 (see FIG. 4) and deflects the laser beam; an fθ lens (a firstscanning lens) 120 which gathers the laser beam in the scanningdirection (a main scanning direction) of the polygon mirror 110 andrenders constant a scanning speed over the photosensitive drum 37; acylindrical lens (a second scanning lens) 140 which gathers the laserbeam in a sub-scanning direction (the rotating direction of thephotosensitive drum 37) orthogonal to the main scanning direction; afirst return mirror 130; and a second return mirror 131.

The laser beam which has been modulated on the basis of imageinformation and emitted from the laser diode 271 passes through orundergoes reflection in sequence of the polygon mirror 110, the fθ lens120, the first return mirror 130, the cylindrical lens 140, and thesecond return mirror 131, as indicted by a dashed line, thereby exposingand scanning the surface of the photosensitive drum 37.

The rigid-body plate (a steel plate in the embodiment) 190 constitutingthe upper surface of the hosing 2 is placed between the side frame 190 bforming the side surface 2 b and the side frame 190 d forming the sidesurface 2 d of the housing 2 (see FIG. 1) at an inner position below thetop cover 18. This rigid-body plate 190 also serves as the upper coverof the scanning unit 100.

The process cartridge 4 includes a drum cartridge 35 and a developmentcartridge 36. The photosensitive drum 37, an charging device 38, and thetransfer roller 39 are provided within the drum cartridge 35. Asmentioned previously, the process cartridge 4 is removably attached tothe inside of the housing 2 by way of the opening appearing when thefront cover 16 is opened. The development cartridge 36 is removablyattached to the drum cartridge 35 and has a development roller 40, alayer thickness regulation blade 41, a feed roller 42, and a tonerhopper 43.

The toner housed in the toner hopper 43 is agitated by means of rotationof an agitator 45 supported by a rotary shaft 44 in the direction ofarrow and discharged from a toner supply port 46 formed in the side ofthe toner hopper 43. The feed roller 42 is rotatably disposed beside thetoner supply port 46. The development roller 40 is rotatably disposed soas to oppose the feed roller 42. The feed roller 42 and the developmentroller 40 remain in contact with each other in such a state where theycompress each other to a certain extent.

The development roller 40 is formed by covering a metal roller shaftwith a roller made of conductive rubber material and is rotated in thedirection of an arrow in FIG. 2 (a counterclockwise direction). Adevelopment bias is applied to the development roller 40. The layerthickness regulation blade 41 is disposed in the vicinity of thedevelopment roller 40. In the layer thickness regulation blade 41, apress section, which has a semi-circular cross-sectional profile and ismade of insulating silicon rubber, is provided at the extremity of ablade main body formed from a metal leaf spring material. The layerthickness regulation blade 41 is supported by the development cartridge36 in the vicinity of the development roller 40, and the press sectionis brought into press contact against the development roller 40 by meansof elastic force of the blade main body.

The toner discharged from the toner supply port 46 is supplied to thedevelopment roller 40 by means of rotation of the feed roller 42. Atthis time, the toner is subjected to positive frictional electrificationbetween the feed roller 42 and the development roller 40. The tonersupplied over the supply roller 40 enters between the press section ofthe layer thickness regulation blade 41 and the development roller 40 inassociation with rotation of the development roller 40, whereby thetoner is carried by the development roller 40 as a thin layer of giventhickness.

The photosensitive drum 37 is supported by the drum cartridge 35 so asto be rotatable in the direction of the arrow (a clockwise direction) ata position beside the development roller 40 in a state of opposing thesame. The drum main body of this photosensitive drum 37 is grounded, andthe surface of the photosensitive drum 37 is formed from apositively-chargable photosensitive layer made of polycarbonate or thelike.

The charging device 38 is located above and to the left of thephotosensitive drum 37, and spaced a predetermined interval from thephotosensitive drum so as to oppose the same. This charging device 38 isof scorotoron type for positive electrification purpose which causes anelectrification wire, such as tungsten, to effect corona discharge. Thecharging device 38 is configured to impart the surface of thephotosensitive drum 37 with a uniform positive charge.

The transfer roller 39 is disposed at a position below thephotosensitive drum 37 so as to oppose the same and is supported by thedrum cartridge 35 so as to be rotatable in the direction of the arrow (acounterclockwise direction). The transfer roller 39 is constituted bycovering a metal roller shaft with a roller made of conductive rubbermaterial. At the time of transfer, a transfer bias is applied to thetransfer roller 39.

In association with rotation of the photosensitive drum 37, the surfaceof the photosensitive drum 37 is imparted with a uniform positive chargeby the charging device 38. Next, the surface of the photosensitive drumis exposed by a laser beam output from the scanning unit 100, whereby anelectrostatic latent image is formed. Subsequently, when thephotosensitive drum 37 opposes the development roller 40 and when thetoner, which is carried by the development roller 40 and positivelycharged, opposes and comes into contact with the photosensitive drum 37,the electrostatic latent image is formed on the surface of thephotosensitive drum 37 by the development bias applied to thedevelopment roller 40; that is, the toner is supplied to the areas onthe surface of the uniformly-positively-charged photosensitive drum 37,which are exposed by the laser beam and whose electric potential islowered, and selectively carried to thus form a toner image (reversaldevelopment).

Subsequently, the toner image carried by the surface of thephotosensitive drum 38 is transferred onto a recording medium by meansof the transfer bias applied to the transfer roller 39 while therecording medium passes between the photosensitive drum 37 and thetransfer roller 39.

The fixing unit 5 is disposed at a position above the paper feedcassette 6, beside the process cartridge 4, and downstream of theprocess cartridge 4 in the transport direction of a recording medium.The fixing unit 5 includes, as fixing rollers, a heating roller 51having an internal heater, and a pressing roller 52 which is disposedopposite the heating roller 51 and forced so as to press the heatingroller.

The fixing unit 5 thermally fixes the toner image, which is a visibleimage transferred to the recording medium by the process cartridge 4,while the recording medium passes between the heating roller 51 and thepressing roller 52. Subsequently, the recording medium is fed to a paperoutput path 76, which is a recording medium transport path formed in thepaper output section 70.

The paper output section 70 includes an inner guide member 71 and anouter guide member 62, which in combination constitute the paper outputpath 76; a lower paper output roller 73 and an upper paper output roller75, which constitute a pair of paper output rollers which are providedat an outlet port used for outputting the recording medium to a sheetoutput tray 72 provided in the top cover 18; and a tray member 74 havinga portion which constitutes a portion of the sheet output tray 72.

The outer guide member 62 constituting the paper output path 76 isconfigured so as to pivot toward a rear surface side of the housing 2 insynchronism with opening/closing action of the rear cover 60 provided onthe rear surface 2 c of the housing 2. When the rear cover 60 pivotallyattached by way of a hinge 61 is opened, the upper portion of the outerguide member 62 pivots toward the rear surface side thereof insynchronism with the opening action of the rear cover 60. Thus, thepaper output path 76 is viewed by way of an opening formed in the rearside surface 2 c of the housing 2 as a result of opening of the rearcover 60.

The sheet output tray 72 has the shape of an essentially-rectangularplate when viewed from above and is configured such that arear-surface-side end portion of the tray is recessed to thus form arecessed portion and such that the rear-surface-side end portiongradually slopes upward from the rear-surface-side end portion towardsthe front side. A region of the sheet output tray 72, which extends fromthe rear-side end portion to an arbitrary point on the upwardly-slopingportion, is formed from the tray member 74. The upper surface of thefront-side (the lead-end-side in the transport direction of a recordingmedium) leading-end portion of the tray member 74 contacts a lowersurface of the side edge portion of the tray member 74 of the top cover18.

The paper traveling direction of the recording medium, which has passedthrough the fixing unit 5 and has been delivered to the paper outputpath 76, is reversed and moved upward by means of the inner guide member71 and the outer guide member 62. The recording medium is delivered tothe pair of paper output rollers. The recording medium is output ontothe sheet output tray 72 toward the front by way of the pair of paperoutput rollers (73 and 75). The rigid-body plate 190 is provided so asto extend across a position immediately below the area of the sheetoutput tray 72 consisting of the top cover 18.

(2) Detailed Configuration of the Scanning Unit 100

The configuration of the scanning unit 100 of the present embodimentwill now be described in detail.

FIG. 3 is a top view for describing the configuration of the scanningunit 100, and FIG. 4 is a schematic cross-sectional view.

The scanning unit 100 has an opening 223 by way of which the laser beamtravels from an area where the first return mirror 130 is mounted (i.e.,the front side) to another area where the second return mirror 131 ismounted (i.e., the rear side). The scanning unit 100 further has aflat-plate-like base section 220 on which individual sections, such asthe polygon mirror 110 and the fθ lens 120, are mounted; and a unitframe 200 having a rim portion 210 forming an outer peripheral wallsurrounding an outer peripheral side of the base section 220 (FIG. 3).The unit frame 200 can be manufactured by integrally forming, e.g.,fiberglass-impregnated resin, through injection molding.

An opening 222 is formed in an area on the base section 220 where thepolygon motor 112 (FIG. 4) for rotating the polygon mirror 110 is to bemounted (FIG. 3). A substrate 260, on which the polygon mirror 112 ismounted, is fastened by screws 261 and 262 from below the opening 222,thereby rendering the scanning unit 100 slim (FIG. 4).

In the area of the base section 220 where the fθ lens 120 and thecylindrical lens 140 are to be mounted, the opening 223 is formed forcausing the laser beam to pass from the upper side of the base section220 (i.e., the part of the base section 220 facing the rigid-body plate190) to the lower side of the base section 220 (the part of the basesection 220 facing the photosensitive drum 37). The scanning unit 100 isalso made slim by arranging the fθ lens 120 and the cylindrical lens 140so as to overlap in the vicinity of the opening 223 when viewed from thetraveling direction of the laser beam from the polygon mirror 110.

Further mounted on the base section 220 are the laser diode 271 foremitting a laser beam, a collimator lens 272 for collimating the emittedlaser beam into parallel light, and an LD holder 270 equipped with aslit plate 273 having slits for shaping the parallel light (FIG. 3). AnLD control substrate 275 for controlling driving of the laser diode 271is mounted on a substrate attachment section 225 which is provided so asto upwardly protrude from the base section 220 toward the inside of therim portion 210 from the base section 220 (FIG. 3). The substrateattachment section 225 is formed integrally with the base section 220 bymeans of injection molding.

The laser beam emitted from the laser diode 271 is deflected by thepolygon mirror 110 to pass through the fθ lens 120 and undergoesreflection on the first return mirror 130 to thus travel downward(toward the lower side) of the base section 220 by way of the opening223. Subsequently, the laser beam passes through the cylindrical lens140 and undergoes reflection on the second return mirror 131 to thuspass through a glass plate 252 attached to the opening formed in a lowercover 250 of the scanning unit 100, thereby exposing the surface of thephotosensitive drum 37 (FIG. 4).

The rigid-body plate 190 that is disposed so as to extend across theinside of the top cover 18 and constitutes the upper surface of thehousing 2 also serves as the upper cover of the scanning unit 100, andthe unit frame 200 is provided so as to hang down from the rigid-bodyplate 190.

FIG. 5 is a perspective view for describing the case where the unitframe 200 is fixed to the rigid-body plate 190.

Holes 188 and 189 formed in the sides of the rigid-body plate 190 arefor use in fixing a positional relationship between the rigid body 190and the side frame 190 b shown in FIG. 1. Although not shown in FIG. 5,similar holes are formed in the side frame 190 d as well.

Holes 181 to 187 are provided in areas of the rigid-body plate 190corresponding to screw-hole pedestals 281 to 287 formed in the unitframe 200. By means of screws 181 a to 187 a inserted into therigid-body plate 190 from above, the rigid body plate 190 and the unitframe 200 are fastened together.

The screw-hole pedestals 281 to 287 provided on the unit frame 200 aresituated at positions on the base section 220 which are located furtherinterior from the rim portion 210. Conventionally, screw holes are oftenformed in the rim portion 210. However, in the present embodiment, thescrew-hole pedestals 281 to 284 are formed on the base section 220.

Each of the screw-hole pedestals 281 to 287 has asubstantially-cylindrical shape, and the screw-hole pedestals 281 to 287are provided so as to project slightly higher than the rim portion 210in the direction of the rigid-body plate 190 (the screw-hole pedestals285 and 286 are illustrated in FIG. 4). As mentioned previously, bymeans of the screw-hole pedestals 281 or the like being provided atpositions on the base section 220 which are further interior from therim portion 210, the unit frame 200 prevents deformation of the overallunit frame 200, which would otherwise be caused when the unit frame 200is fastened to the rigid-body plate 190.

The unit frame 220 is formed by means of injection molding. The upperedge of the rim portion 210 of the unit frame usually may be formed withdeteriorated precision and become wavy. However, the precision of thecylindrical screw-hole pedestals is comparatively high, and hence thecylindrical screw-hole pedestals are easily brought flush with eachother. Accordingly, the screw-hole pedestals are made higher than therim portion 210, and the screws are engaged into the screw-holepedestals. Thereby, when compared with a case where screw holes areformed in the rim portion, the fixed positions become closer to thepolygon mirror, which acts as a source of vibration, thereby hinderingoccurrence of vibration.

When the unit frame 200 is fastened to the rigid-body plate 190, asponge (not shown) is interposed between the rim portion 210 and therigid-body plate 190, thereby preventing intrusion of dust into thescanning unit 100.

The rims of the screw holes of the screw-hole pedestals 281 to 287 aremade essentially identical in thickness with the individual portions,such as the rim portion 210 and the base section 220. When the unitframe 200 is cooled after injection molding, the periods of timerequired to cool the individual sections are made essentially equal toeach other, thereby improving mass-productivity of the unit frame 200.

The three screw-hole pedestals 285 to 287 are disposed around thepolygon motor 112 for driving the polygon mirror 110. In the embodiment,in order to make the unit frame 200 slim, the opening 222 is provided inthe position where the polygon motor 112 of the base section 220 isprovided (FIG. 3). The substrate 260, on which the polygon mirror 112 ismounted, is fastened by the screws 261 and 262 from below the opening222 (FIG. 4). In such a case, particularly, vibration attributable tothe polygon mirror 112 presents a problem. When the rigid-body plate 190is fastened to the unit frame 200, at least three screw-hole pedestalsare provided around the polygon mirror 112, thereby suppressing thevibration.

The screw-hole pedestals 285 to 287 provided at three areas around thepolygon mirror 112 are preferably located at positions surrounding thedrive shaft (the rotary shaft of the polygon mirror 110) of the polygonmotor 112, in a triangular pattern formed from the three pointscorresponding to the three areas where the screw-hole pedestals areprovided. However, as a matter of course, hindrance of the path of thelaser beam that has been deflected by the reflection surface of thepolygon mirror 110 and travels toward the fθ lens 120 and the returnmirror 130 is not preferable. Therefore, the screw-hole pedestal 285provided in the direction of the fθ lens 120 is disposed in the vicinityof a non-use area of the fθ lens 120 (the area of the lens through whichno laser beam passes).

When the polygon mirror 110 is used as a deflector, if the distancebetween the polygon mirror 110 and the screw-hole pedestals 285 to 287is too short, whistle-like sound may arise between the polygon mirror110 and the screw-hole pedestal 285 during high-speed rotation of thepolygon mirror 110. The locations where the screw-hole pedestals are tobe provided are preferably determined within the range where no suchsound arises. From the viewpoint of prevention of vibration, thepositions where the screw-hole pedestals 285 to 287 are to be placed arepreferably spaced at substantially equal distances from the drive shaftof the polygon motor 112 so as to become more analogous to anequilateral triangle.

FIG. 6 is a perspective view for describing how the rear cover 250 isattached to the back side of the unit frame 200.

This drawing illustrates how the substrate 260 on which the polygonmotor 112 is to be mounted is fastened from the back of the base section220 by means of the screws 261 and 262 and screws 261 a and 262 a.

A pair of attachment sections 141 a, 141 b to be used for mounting thecylindrical lens 140 are formed integrally with the unit frame 200 inthe vicinity of the opening 223, by means of injection molding. Thecylindrical lens 140 is mounted by means of pieces of attachment clips142 a and 142 b.

The rear cover 250 has an opening through which the light beam passestoward the photosensitive drum 37, and the glass plate 252 is attachedto the opening. The rear cover 250 is formed from a rigid plate (a steelplate in the embodiment) which is thinner than the rigid-body plate 190.As mentioned above, deformation of the unit frame 200, which wouldotherwise arise when the unit frame is fastened to the rigid-body plate190, is also prevented by making the strength of the rear cover 250lower than that of the rigid-body plate 190. The rear cover 250 is notnecessarily formed from a steel plate. A cover formed from, e.g., resin,can also be used as a rear cover, so long as the cover is lower instrength than the rigid body plate 190.

In the scanning unit 100 of the present embodiment, an angle a(hereinafter called a “return angle”) between the laser beam enteringthe first return mirror 130 and the laser beam reflected from the firstreturn mirror 130 is an acute angle of about 30° (FIG. 4). By means ofmaking the return angle a small, the laser beam reflected from the firstreturn mirror 130 is guided directly to the second return mirror 131,thereby slimming down the scanning unit. The return angle can be set toabout 10° to 30°. The smaller return angle is advantageous in slimmingdown the scanning unit 100.

The first return mirror 130 and the second return mirror 131 aredisposed in the vicinity of the outer peripheral wall. The second returnmirror 131 is configured to be disposed at a position below the polygonmirror 110, by way of the base section 220. Specifically, the polygonmirror 110 and the second return mirror 131 are disposed at one of twomutually-opposing ends of the base section 220, and the first returnmirror 130 is disposed at the other end of the two ends with the opening223 interposed therebetween.

Given that an optical path length from a reflection surface of thepolygon mirror 110 at the center of the optical beam in a main scanningdirection to a reflection surface of the first return mirror 130 istaken as “a”; another optical path length from the reflection surface ofthe first return mirror 130 to a reflection surface of the second returnmirror 131 is taken as “b”; and still another optical path length fromthe reflection surface of the second return mirror 131 to the surface ofthe photosensitive member 37 is taken as “c,” a relationship of a<b andanother relationship of c<b are satisfied. By means of placing thesecond return mirror 131 at a position below the polygon mirror 110, theoptical path length “b” from the reflection surface of the first returnmirror 130 to the reflection surface of the second return mirror 131 canbe ensured long. Therefore, assurance of the long optical path length“b” is effective for shortening a distance between the scanning unit 100and the photosensitive drum 37. This can contribute to miniaturizationof the overall image forming apparatus.

In order to slim down the scanning unit 100, the thickness of thecylindrical lens 140 in the sub-scanning direction is slimmed down toabout 6 mm. When viewed in a direction from the polygon mirror 110 tothe first return mirror 130, the fθ lens 120 and the cylindrical lens140 are arranged in such a positional relationship that they partiallyoverlap each other in a direction parallel to the axis of the rotaryshaft of the polygon mirror 110 (FIG. 4). As mentioned above,arrangement of the cylindrical lens 140 in the area of the opening 223(particularly a position closer to the first return mirror 130 than tothe fθ lens 120) contributes to slimming down of the cylindrical lens140 as well. As the cylindrical lens 140 is located closer to thephotosensitive drum 37, the thickness of the cylindrical lens 140 in thesub-scanning direction is increased, thereby posing difficulty inslimming down of the cylindrical lens.

FIG. 7 is a perspective view for describing how the cylindrical lens 140is attached to the rear surface of the unit frame 200.

A pair of attachment sections 141 a, 141 b to be used for attaching thecylindrical lens 140 are formed in the vicinity of the opening 223 andintegrally with the unit frame 200 by means of injection molding. Thecylindrical lens 140 is attached by means of pieces of attachment clips142 a, 142 b.

The attachment clip 142 a has a section formed into the shape of a leafspring as a leg section 1421 a, and the attachment clip 142 b has asection formed into the shape of a leaf spring as a leg section 1421 b.The leg sections 1421 a, 1421 b of the pieces of the attachment clip 142a, 142 b are interposed between the attachment sections 141 a, 141 b atthe respective ends of the cylindrical lens 140 with the respective endsof the cylindrical lens 140 being loosely attached to the attachmentsections 141 a, 141 b, thereby fixing the cylindrical lens 140 to theback of the base section 220.

The cylindrical lens 140 is formed from resin through injection molding.A gate section 144 is left on one end which comes into contact with theresin injection section when the lens is removed from a molding die. Inthe meantime, a positioning section 145 to be used for positioning thecylindrical lens 140 is provided in an attachment section 141 a formedin the position opposite that of the gate section 144. As a result ofthe end opposite to the gate section 144 is being provided so as to comeinto contact with the positioning section 145, the cylindrical lens 140is positioned.

In general, the cylindrical lens 140 has hitherto been positioned in thecenter with respect to the main scanning direction. In the scanning unit100 of the embodiment, an opening 223 is formed in a position on thebase section 220, where the cylindrical lens 140 is to be attached, forminiaturization purpose. This makes it difficult to position thecylindrical lens 140 in the center with respect to the main scanningdirection. In contrast, when the positioning sections are provided atboth ends, there arises apprehension that a characteristic of thecylindrical lens is deteriorated when the lens becomes inflated forreasons of moisture absorption, a temperature hike, or the like.

In the present embodiment, the cylindrical lens 140 is positioned bymeans of only the end section opposite to the gate section 144, wherebythe lens is positioned while the influence of inflation of the lens isminimized. The attachment clip 142 b used for fastening the cylindricallens 140 assumes a shape which allows inflation of the lens in the mainscanning direction (deformation of the lens due to inflation or the likeis not inhibited by a leaf spring 1421 b). Deterioration of thecharacteristic of the lens and fracture of the lens can be prevented bymeans of the shape of the attachment clip 142 b.

As has been described above, the scanning unit of the present embodimentenables making of an attempt to miniaturize the scanning unit whilerequired optical path lengths are ensured within the scanning unit. Anattempt can be made to slim down the scanning unit and miniaturize theimage forming apparatus by means of shortening the distance between thescanning unit and the photosensitive member which is an object ofscanning. In the scanning unit of the present embodiment, acircumferential wall assumes an essentially-wedge-shaped geometry whenviewed in sideways. As shown in FIG. 2, the front cover 16 side of thescanning unit 100 is further slimmed down. Hence, when the front cover16 is opened, a space which enables passage of the process cartridge 4is easily formed in a lower portion of the scanning unit 100, therebyenabling making of an attempt to slim down the image forming apparatus.

(Modification)

Although the embodiment of the present invention has been described thusfar, it goes without saying that the present invention is not limited tothe specific example described in connection with the embodiment. Forinstance, the following modification can also be performed.

(1) The embodiment has described a configuration in which the threescrew-hole pedestals 285 to 287 to be provided around the polygon motor112 are provided at three positions around the polygon motor 112.However, any number of positions may be employed, so long as the numberis equal to three or more. Moreover, this embodiment has also describeda configuration in which one screw-hole pedestal 285 is provided at aposition around the polygon motor close to the f lens 120. However, twoscrew-hole pedestals can also be provided at two positions around thepolygon motor close to the fθ lens 120. In this case, as a matter ofcourse, the two screw-hole pedestals are disposed in a nonuse area ofthe fθ lens 120 (i.e., two locations; that is, right and left positions,when viewed from the optical axis of the fθ lens 120).

(2) The embodiment has described a configuration in which the laser beamreflected from the first reflection mirror 130 is caused to directlyenter the second return mirror 131 provided on the back of the basesection 220. Employment of such an optical path is suitable forminiaturizing the scanning unit 100. However, there may also be employeda configuration where an additional return mirror is interposed betweenthe first return mirror 130 and the second return mirror 131.

(3) In the embodiment, the opening 222 is formed in the position on thebase section 220 where the polygon mirror 110 and the polygon motor 112are mounted. Although this configuration is suitable for making thescanning unit 100 slim, the substrate on which the polygon motor 112 isto be mounted may be provided on the front side of the base section 220without forming the opening 222. Even when the substrate is provided onthe front side of the base section 220, there can be yielded anadvantage of miniaturization resulting from the scanning unit 100hanging from the rigid-body plate 190.

Also, even when the substrate is provided on the front of the basesection 220, an advantage of miniaturization may be yielded by means offorming the optical path of the laser beam into such a shape as shown inFIG. 4 through use of the first return mirror 130 and the second returnmirror 131.

(4) Although the above-described embodiment has used the polygon mirror110 and the polygon motor 112 as a deflector for deflecting a laserbeam, the deflector is not limited to them. For instance, a galvanomirror or the like can also be used as the deflector. In this case, thescanning lens is not limited solely to the fθ lens 120, but use of alens of another optical characteristic is also conceivable.

(5) The above embodiment has described attachment of the cylindricallens 140. As a matter of course, the similar attachment method can beapplied to the fq lens 120. More specifically, an attachment section tobe used for attaching both ends of the fq lens 120 are provided on thefront side of the unit frame 200. A space between both ends of the fqlens 120 and the attachment section can fixed through use of theattachment hardware.

The present invention can be applied to an image forming apparatusequipped with a scanning unit which emits a light beam and scans anobject of scanning, such as a photosensitive member, as well as to ascanning unit.

1. An image forming apparatus comprising: a photosensitive member; ascanning unit that exposes and scans a surface of the photosensitivemember by emitting a light beam, the scanning unit including a unitframe that has a base section having an opening and an outer peripheralwall surrounding a rim of the base section; a deflector that is disposedon an upper surface side of the base section and deflects the light beamtoward the outer peripheral wall; a first reflection optical elementthat is disposed with the opening being interposed between the firstreflection optical element and the deflector and reflects the lightbeam, which has been deflected by the deflector and travels toward theouter peripheral wall, toward a position below the base section; and asecond reflection optical element that is disposed on a lower surfaceside of the base section and reflects the light beam, which travelsafter having been reflected by the first reflection optical element andpassed through the opening, toward the photosensitive member; wherein anangle between the light beam entering the first reflection opticalelement and the light beam reflected from the first reflection opticalelement toward the second reflection optical element is an acute angle;and a relationship of a<b and another relationship of c<b are satisfied,given that an optical path length from a reflection surface of thedeflector to a reflection surface of the first reflection opticalelement is taken as “a”; another optical path length from the reflectionsurface of the first reflection optical element to a reflection surfaceof the second reflection optical element is taken as “b”; and stillanother optical path length from the reflection surface of the secondreflection optical element to the surface of the photosensitive memberis taken as “c.”
 2. The image forming apparatus according to claim 1,wherein the deflector is disposed at one of two opposing ends of theupper surface of the base section; the first reflection optical elementis disposed at the other end of the two ends; and the second reflectionoptical element is disposed via the base section at the one end of thetwo ends and a position below the deflector.
 3. The image formingapparatus according to claim 1, wherein the scanning unit comprises: afirst scanning lens interposed between the deflector and the firstreflection optical element; and a second scanning lens interposedbetween the first reflection optical element and the second reflectionoptical element.
 4. The image forming apparatus according to claim 3,wherein the deflector has a rotary shaft about which the reflectionsurface of the deflector rotates to deflect the light beam; the openingis formed in an area where the first scanning lens and the secondscanning lens are disposed; and the first scanning lens and the secondscanning lens are provided so as to partially overlap each other in theopening along a direction of the rotary shaft.
 5. The image formingapparatus according to claim 3, wherein the second scanning lens is acylindrical lens that gathers a light beam in a sub-scanning directionintersecting a main scanning direction.
 6. The image forming apparatusaccording to claim 5, wherein a thickness of the cylindrical lens in thesub-scanning direction is about 6 mm or less.
 7. The image formingapparatus according to claim 1, wherein an angle between the light beamentering the first reflection optical element and the light beam havingbeen reflected from the first reflection optical element is about 10° to30°.
 8. The image forming apparatus according to claim 4, wherein bothends of the second scanning lens are respectively attached to twoattachment sections provided on a lower surface of the base section byattachment members.
 9. The image forming apparatus according to claim 8,wherein a positioning section of the second scanning lens is provided oneither one of the two attachment sections.
 10. The image formingapparatus according to claim 9, wherein the second scanning lens isformed from resin by injection molding and has a gate section that isleft in a resin injection part after molding is performed at one end ofthe second scanning lens; and the positioning section is provided on theattachment section to which an end opposite to the one end of the secondscanning lens is to be attached.
 11. The image forming apparatusaccording to claim 1, wherein the scanning unit has a substantiallywedge-shaped geometry whose part close to the deflector is longer than apart close to the first reflection optical element in a heightwisedirection in a side view of a rim portion of the unit frame.
 12. Theimage forming apparatus according to claim 11, further comprising aprocess unit including the photosensitive member and a developmentdevice for rendering visible a latent image formed by exposing andscanning the photosensitive member; and a cover capable of being openedand closed which is provided on a side surface of a housing on a firstreflection optical element side; wherein the process unit is removablyattached to the image forming apparatus through an opening formed whenthe cover is opened.
 13. The image forming apparatus according to claim1, wherein the optical path length “a” is a length from a reflectionsurface of the deflector that is located at a center of said opticalbeam in a main scanning direction to a reflection surface of said firstreflection system optical element is taken as
 14. A scanning unit thatexposes and scans an object of scanning by emitting a light beam,comprising: a unit frame that has a base section having an opening andan outer peripheral wall surrounding a rim of the base section; adeflector that is disposed on an upper surface side of the base sectionand deflects the light beam toward the outer peripheral wall; a firstreflection optical element that is disposed with the opening beinginterposed between the first reflection optical element and thedeflector and reflects the light beam, which has been deflected by thedeflector and travels toward the outer peripheral wall, toward aposition below the base section; and a second reflection optical elementthat is disposed on a lower surface side of the base section andreflects the light beam, which travels after having been reflected bythe first reflection optical element and passed through the opening,toward the photosensitive member; wherein an angle between the lightbeam entering the first reflection optical element and the light beamreflected from the first reflection optical element is an acute angle;and a relationship of a<b is satisfied, given that an optical pathlength from a reflection surface of the deflector to a reflectionsurface of the first reflection optical element is taken as “a” andanother optical path length from the reflection surface of the firstreflection optical element to a reflection surface of the secondreflection optical element is taken as “b.”